Ultrafast Intermolecular Vibrational Energy Transfer in Hexahydro-1,3,5-trinitro-1,3,5-triazine in Molecular Crystal by 2D IR Spectroscopy

Cyclic nitramine hexahydro-1,3,5-trinitro-s-triazine (HHTT) is an important energetic molecule with a variety of applications. In this work, structural dynamics and vibrational-energy transfer dynamics of HHTT were investigated using steady-state infrared (IR), femtosecond time-resolved IR, and two-dimensional (2D) IR spectroscopic methods. Two different structural forms of HHTT (isolated molecules dissolved in DMSO and microcrystal dispersed in paraffin oil) were examined using the NO2 asymmetric stretching mode as a structural marker. Both intra- and intermolecular vibrational energy transfers were observed in HHTT microcrystal, where the well-known α-conformation was retained. However, less perfect α-HHTT species was found in DMSO. An intermolecular energy dissipation pathway was found to be related to Davydov splitting in the microcrystal form of HHTT, which originates from intermolecular interactions (e.g., electrostatic and hydrogen bonds). Experimental results were supported by quantum-chemistry co...